Boost Is King: Can Your Head Gaskets Handle It?

In today’s enthusiast marketplace, boost equals power. Boost has always been an easy path to ultimate high performance. It was just a little bit more black magic years ago to properly tune a boosted engine than it is today. Having the correct technical knowledge is key to making the most reliable power with boost. Because whether you’re boosting your street car, or trying to blow your heads off at the track, boost is king! But boost–big or small–must be contained within the engine in order to improve performance, and it’s companies like SCE Gaskets who make this possible through the use of their time-tested head gaskets.

While a lot of races are won using boost, lack of control is a big reason that a lot of races are lost because of boost. Blown head gaskets. Blown intake gaskets. Blown hoses and clamps. Even stretched or broken cylinder head fasteners can all lead to boost losses and lost races. But we know there are people out there running double- and even triple-digit boost figures in their engines, so how are they keeping it all in?

Copper head gaskets offered by companies like SCE give the end-user the ability to seal up forced induction pressures with ease.

Cylinder Pressure Limitations

Whether you’re racing down the quarter-mile, or sitting behind the wheel of a big-rig for a long haul, the head gaskets sealing your cylinders were designed to function within specific operating environments only. If you’re boosted, then the higher combustion pressures in your cylinders dictate the use of higher-strength materials and better construction than run-of-the-mill OE gaskets. Boosting an engine effectively raises its final compression ratio, which is the engine’s static compression ratio plus the extra boost factored in. So that means that your boosted engine will require head gaskets with sealing capabilities of a high compression race engine, even if you’re running low compression on the street.

The formula I use to calculate what I like to refer to as the “Final Compression Ratio” or FCR is:

((Boost / 14.7) + 1) x Static Compression = FCR

Let’s say you have an engine with 9.0:1 static compression ratio, and you run 8 psi boost, your FCR will be 13.9:1.

((8/14.7) + 1) = 1.544
1.544 x 9.0 = 13.9

So you should choose a head gasket designed to seal continuously at around 13.9:1 CR or higher. And the more boost you make, the harder it is for the head gaskets hold it in. That’s when you need to add a mechanical advantage within your engine to help your head gaskets do their job. Over the years, stainless-steel wire O-rings pressed into very small precisely cut grooves surrounding the cylinder bores have been the tried-and-true method of assisting head gasket seal. But “O-ringing” a block, or heads, requires that you also cut a shallow receiver groove into their mating parts. Otherwise your O-ring will just be crushing the gasket, which can be partially effective and possibly more effective than no O-rings at all. But it’s still not quite as good as the correct O-ring and matching receiver groove set-up. But O-rings require a specific type of head gasket too. They should only be used with copper head gaskets.

These two graphics show the specifics of wire ring installation to ensure maximum performance achievement.

Conformability Is Key

Steel is stronger than copper, but copper is much more conformable than steel.

I once visited with a guy who was dropping off his block and heads at a machine shop yet again, for what seemed like the hundredth-time. He said he keeps blowing head gaskets and he’s even tried the very best MLS (multi-layer steel) gaskets money could buy. I mentioned copper gaskets. He replied that he never tried copper because he reasoned that steel is stronger than copper. And while he may be technically correct–steel is stronger than copper–but when making extreme performance head gaskets that will last, it is not just about the strength of the material. It’s more about the conformability of it. And copper is much more conformable than steel.

Here’s a shot of properly-installed O-rings pressed into a set of Dart Pro 1 heads for a 2,500 horsepower engine.

In order to stay sealed under the highest cylinder pressures, you need a material that can conform to the impurities of the sealing surfaces. Especially when the gaskets start to get heated up and pushed around. Composite gaskets can do a pretty good job of conforming, but they’re not very strong. Copper is much stronger than any composite gasket material, yet it’s still malleable, meaning copper can stretch and conform to remain sealed. Kind of like “give a little, take a lot.” Copper might even sacrifice itself to the mighty boost gods every now and then, but it’ll hold up much longer than its composite counterparts. And it’ll get you home, or into the winner’s circle safely.

This is the aluminum block with the correctly-machined receiver grooves cut into the steel cylinder liners.

SCE has compiled a series of charts outlining the baseline limitations for each type of head gasket used in normally aspirated, supercharged (aka boosted), and nitrous oxide-injected engines. These are only guidelines, as actual engine parts and combinations (FCR, camshaft specs, type of fuel used, etc.) will always have an impact on ultimate combustion pressures.

Notice how in the boosted and N2O charts MLS gasket performance degrades much quicker than copper.

Pressure Conversions

Choosing the right head gaskets and properly tuning a boosted engine requires just a bit more than the most basic understanding of pressure. But pressure is measured, and/or represented, in all sorts of different ways around the world. Even on my own dyno I sometimes have trouble converting boost numbers into a format my brain is accustomed to seeing, which has always been Pounds-per-Square-Inch (PSI) for me. But boosted PSI inside an engine is more accurately represented as PSIG, (aka Gauge Pressure, see chart), when tuning. And gauge pressure is different from PSIA, which is “Absolute” or “Actual” pressure. PSIA is boost + atmospheric pressure. And since atmospheric pressure can change from run-to-run, and from day-to-day, it’s important to know that it will affect the true, overall PSI your engine will see inside its runners.

Here’s a chart I made up to help converting all the ways I have seen an engine’s boost measured. There may be more ways that I’m unaware of, so please feel free to let me know if I’m missing any. I also threw vacuum (inHg) into the chart, and I use this pretty regularly when tuning different engines and EFI systems on my dyno since no two system’s software may use the same values. One EFI tuning software might show kPa. One might use PSIA. One could show BAR. I have to be able to know what’s what when tuning these damn things and it can get confusing sometime. So here’s my chart. Also, feel free to let me know if I’ve made any errors. I’m not a math or scientific wiz. I just fed all this data I found on the internet into my Excel spread sheet and it spit out this chart. So it must be true. Right?

KEY:

PSI = kPa × 0.145037738

inHg = kPa x 0.2952998751

BAR = kPa x 0.01

PSIG = PSIA – 14.7

PSIG = Pressure Gauge (Boost inside the engine as seen on a typical gauge)

PSIA = Pressure Absolute (Boost + Atmospheric Pressure)

While it’s also possible to install the O-rings in the block, and cut receiver grooves into the heads. This method is not ideal when running aluminum heads due to the deformation of the grooves in the softer aluminum.

Below is a list of the three types of copper head gaskets manufactured by SCE; each one is optimized for a different usage scenario. For the best recommendation for your particular engine, contact SCE directly.

To keep from lifting the cylinder heads, high-boost or N2O race engines are often built with larger diameter cylinder head studs. If you’re planning to O-ring an engine fitted with larger head studs, you must consider the copper gasket’s clamping area too. See the photo and caption below for more details.

On the left is an SCE Titan copper head gasket with a 4.570-inch bore diameter and .531-inch bolt holes to clear ½-inch head studs (PN T13574). On the right is SCE’s larger 4.600-inch bore gasket but with smaller .480-inch holes that will only clear 7/16-inch head studs (PN T13624). Note how the material remaining between the bore and head stud hole is close to the same width in either gasket. If you tried to run a ½-inch heads stud in the 4.600-bore gasket on the right, the gasket margin would be too thin to maintain a good O-ring seal. Luckily, SCE won’t even sell you such a head gasket, so you don’t really have to worry about making this mistake. And don’t even think about drilling one out yourself. It’s just good knowledge to have.

SCE’s Ryan Hunter walked us through these tips for successfully running copper head gaskets.

What’s the best way to prepare the block and cylinder head? Use a residue-free solvent such as aerosol brake cleaner and a clean rag on the head and block sealing surfaces before assembly. Of course the block and head should be flat within .002-inch across and .004-inch lengthwise, with surface finish of 60 to 80RA preferred, 60 to 100RA acceptable.

Are sealants required? Yes, some method of liquid sealing is required if the engine will be running coolant or oil through the head gasket. Some racing specific engines either do not run coolant or re-route the coolant and oil away from the head/block mating surfaces, so no additional sealants would be required in those applications. You don’t need very much sealant. But don’t use silicone. People get into trouble with leaking head gaskets when they use too much sealant, especially too much silicone. Since the block and head surfaces are flat, the potential leak paths are very small, even with a 100RA surface finish the peaks and valleys are only about .002-inch, which doesn’t require very much sealant to be fluid-tight. Head gasket dressings do not cure, therefore, as the head bolts are tightened the sealant ‘flows’ from the places it’s not needed (peaks), but remains in place to seal the leak paths (valleys). By contrast, silicone cures to form a layer that the cylinder head can sit on. We recommend and use Copper Coat and Hylomar in aerosol cans, simply spray a light coat on both sides of the gasket, let it ‘tack up’ for a while (no less than 2 hours), and you’re ready to bolt the heads on. Silicone, weather-strip adhesive, or any other rubber-like sealants or coatings, should never be placed near the combustion seal in performance engines.

Multi-Layered-Steel, (MLS), head gaskets are sealing most factory engines today. And they can seal boosted engines too. But their performance degrades faster than copper when subjected to extremely high boost or N2O levels.

What about re-torquing? Solid copper does not compress, it displaces. Since the copper gasket body does not compress no re-torque is technically necessary. However, since an engine built using a copper head gasket is almost always within the realm of extreme performance, SCE recommends a re-torque after a complete heat cycle.

Do copper head gaskets require different torque values? Generally, no. Fastener torque values are determined in relationship with the cylinder head and block structure, and the yield strength of the fastener. Arbitrarily increasing torque values could distort the block or head or cause a fastener to fail. However, there are good cases for fine tuning the torque values based upon how the head gaskets look after the first use. A nice thing about copper head gaskets is that they’re reusable and you can ‘read’ them very easily once you know what to look for. You want to see evenly distributed clamp load. Gaskets require the proper clamp load to do what they do and copper gaskets can tell you where the clamp load is light by keeping their shine. Specifically, you want to see machining marks from the block and head surfaces transferred to the copper everywhere on the gasket. Places where the original smooth finish of the copper remains needs some attention. Keep in mind there may be other factors in play such as an O-ring receiver groove that has become too shallow from surfacing, or a head fastener bottoming on the threads giving a false torque reading. Once you have eliminated any mechanical obstruction preventing the head from seating properly, you can safely increase torque values in areas showing light load by 5 to 10 lb-ft.

I took this photo for a story about cylinder head studs, but it caught a great example of how copper head gaskets can help you ‘read’ clamp load around the bores. Notice the area on the gasket directly below the stud removal tool clearly shows the machining marks transferred from the cylinder head surface. If those marks were missing, that would indicate not enough clamp load in that area.

How does the use of copper gaskets help the tuner? Copper is the standard for electrical conductors. In head gaskets we don’t care too much about electricity but we do deal with heat. Superior conductivity benefits engines in two primary ways: Block & head temperatures are more even. And combustion chamber hot spots are dissipated quickly. Cylinder block/head temperature parity is an aid to tuning, though frankly, it’s a minimal factor until you reach the narrow end of the tuning window. The big advantage of conductivity is in the combustion chamber area. In and around the combustion chamber standard composite head gaskets and MLS head gaskets are somewhat insulated from the cylinder head and block by the facings and coatings on the gasket. Heat related failures occur more often with composite and MLS head gaskets than they do with copper because the heat is trapped within the gasket body allowing hot spots to intensify, whereas copper being both a better conductor and having direct metal-to-metal contact with the block and head transfers the heat more effectively.

What do you mean when you talk about elasticity? Another interesting feature of copper, this benefit comes into play when you’re out of the tuning window far enough to actually damage a normal head gasket. Un-alloyed or pure copper has a 25-percent coefficient of elasticity. Meaning in a 4-inch section, the copper head gasket can stretch to 5-inches before it ruptures. This gives the user a ‘safety factor’ not available with other head gasket materials. Blown, nitrous or turbocharged engines can develop cylinder pressures high enough to lift the cylinder head or push the gasket out. If this happens with copper the damage will be apparent but the head gasket hasn’t yet failed. The safety factor of elasticity allowed the copper gasket to push out but still remain intact so you can either back it down and make the next round or back it down and drive home. If you push a composite gasket, game over.

What are some of the specifics of O-ring sealing? Head gasket sealing is a matter of balance and more pressure is needed around the combustion seal than other areas of the gasket. This is due to the vast difference in pressures acting against the head gasket. Consider that an engine developing 1.5 to 2 horsepower per cubic inch will have combustion chamber pressures well over 1,000-psi. While, less than one half-inch away, the cooling system is just running at only 15-22psi max. Since a standard copper gasket is flat, clamp load from the head bolts will be distributed evenly unless some method is used to ‘tip the balance’ and concentrate more load around the combustion sealing area. The accepted method has been to install O-rings on one component, and receiver grooves in its mating part, to accept the O-ring.

Here’s a good tip for installing all types of head gaskets on engines running head studs. Put the gaskets on the block first. Then screw in the fasteners. Trying to get head gaskets down over head studs is a very quick path to ruined gaskets and frayed nerves.

In Conclusion

When you’re building an engine with a large amount of boost — whether the pressure comes from a supercharger or a turbocharger — having a head gasket which seals that boost into the engine is a critical step of the build process. There are many factors which come into play, and perhaps the most important is selecting the correct product for the application.

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About the author

Michael Petralia

Mike is a veteran from Weiand Automotive Industries, where he spent 9 years testing & developing many products. He later moved to the editorial trade, holding titles at several high performance magazines. In 2006, Horsepower TV's Tech Producer position moved him to Tennessee where he later opened Hardcore Horsepower, LLC, building cars and engines for magazines and customers alike. Mike's shop has a 2,000hp engine dyno, a 1,200cfm flowbench, and a 1,250hp chassis dyno giving him unparallelled testing & tuning capabilities.